Cosmo Command cartridges and cases from GBMake.com
I wanted to test out using the rumble feature in Cosmo Command since it's such a recurring feature of the Star Fox games (famously in 64, but also for Command). I saw that GBMake.com had rumble carts as an option, so I made an order with them just to see how things went.
I'm not exactly a professional in these matters, but the cartridges seem to look and work great. Rumble is definitely something you can feel, and they even made cartridge labels and cases for me based on my files. Pretty convenient! If you get a transparent shell, you can see their boards are custom printed with the GBmake logo, which feels a little premium. I didn't do a lot of shopping around, but the prices seemed good to me too.
I probably won't need more carts for a while, but I would definitely order from them again.